Wikipedia
Cryptodonta
The Cryptodonta are a nearly-extinct subclass of the bivalves. It contains a single extant order, Solemyoida, while the Praecardioida are known only from fossils.
The valves of the shell are relatively thin and somewhat elongated. Unlike most other bivalves, species in this group have no hinge teeth on their shells. They have relatively primitive, "protobranchiate", gills.